A semiconductor device having an insulated gate bipolar transistor (IGBT) cell and a free wheeling diode (FWD) cell in the same semiconductor substrate has been proposed, for example, in JP 2007-214541A corresponding to U.S. 2007/0170549.
In the semiconductor device, for example, a P-type layer is formed in a surface portion of an N-type semiconductor substrate, and an N-type emitter region is formed in a surface portion of the P-type layer. Further, first trenches are formed to reach the N-type semiconductor substrate while passing through the N-type emitter region and the P-type layer. A gate electrode is embedded in each of the first trenches through an insulating film.
Further, a P+-type region deeper than the N-type emitter region is formed between the adjacent first trenches for a contact. A second trench is formed to reach the P-type layer while passing through the P+-type region. An emitter electrode is formed above the N-type semiconductor substrate through an interlayer insulating film that covers the gate electrodes. The emitter electrode is embedded in the second trench. That is, the first trenches are configured to form a trench gate structure, and the second trench is configured to form an emitter contact.
On the rear surface of the N-type semiconductor substrate, a P+-type collector region and an N+-type cathode region are formed. Further, a collector electrode is formed commonly on the P+-type collector region and the N+-type cathode region. In such a structure, a section including the P+-type collector region serves as an IGBT element, and a section including the N+-type cathode region serves as a diode element.
In the section of the diode element, that is, in a diode cell, the emitter electrode embedded in the second trench serves as an anode electrode of the diode cell. The inside of the P-type layer to which the anode electrode is connected has an impurity concentration lower than that of the P+-type region. When the diode cell is operated, excessive hole injection from the IGBT cell to the diode cell is restricted. As a result, a recovery characteristic of the diode cell is improved.
In the semiconductor device, however, since the second trench passes through the P+-type region and reaches the P-type layer, between the adjacent first trenches. Therefore, it is necessary to form the P+-type region deeper than the N-type emitter region so that the P+-type region can contact the emitter electrode formed in the second trench. Further, since the P+-type region is located between the first trench and the second trench, a clearance of the P+-type region for reducing an influence on a threshold voltage Vt of the IGBT is very small.
Such a drawback also arises in a semiconductor device having a P-type semiconductor substrate.
Further, in the semiconductor device of JP 2007-214541, the P+-type region that contacts the emitter electrode in the diode cell has an impurity concentration to determine the threshold voltage Vt of a channel region of the IGBT cell. The impurity concentration is too high as the impurity concentration of the anode of the diode element.
Therefore, JP 2007-214541A also proposes to form a P-type anode region with an impurity concentration lower than that of the P-type region of the IGBT cell without forming the first trenches and the second trench in the diode cell. The P-type anode region is formed by using a separate mask in a special process. In such a structure, hole injection to the semiconductor substrate is restricted, and hence a desirable diode characteristic can be achieved.
In the structure having the special P-type anode region in the diode cell, however, hole injection from the IGBT cell to the diode cell relatively increases. As a result, a forward voltage Vf will be shifted, and a reverse recovery capacity will be reduced. Further, a sectional structure of the diode cell is different from that of the IGBT cell. Therefore, an electric field is likely to concentrate adjacent to the bottom of the trench located at an end of the IGBT cell, resulting in the degradation of withstand voltage.